In recent years, automatic recognition, information management, and traceability management using radio frequency identification (RFID) have been spread. RFID systems are used in various industrial fields because data is writable, large amount of information can be processed, and long distance communication is allowed. Frequencies used in RFID systems include HF band (mainly 13.56 MHz) and UHF band (860 MHz to 960 MHz). RFID systems using UHF band are becoming smaller component and can be used for longer distance communication because the used frequency is high (wavelength is short).
An RFID system includes a reader/writer (R/W) and an RFID tag, each including an antenna. The reader/writer modulates radio frequency (RF) carrier signals corresponding a command and transmits the modulated RF carrier signals. The RFID tag receives signals from the reader/writer and generates a DC power from an RF carrier signal. The RFID tag then drives a circuit by the generated DC power, interprets the received command, and performs a process corresponding to the command. When the command requests a response, the RFID tag generates binary data by reflecting or absorbing the received RF carrier signal by the modulation circuit based on the data generated within the RFID tag and responses with the generated binary data.
In a typical RFID system, the distance between a reader/writer and an RFID tag is not defined, and an RFID system is used at various distances depending on customers' use. Thus, one of requirements for an RFID tag is that stable communication is possible regardless the distance between a reader/writer and the RFID tag, in other words, the dynamic range thereof is wide. Therefore, an RFID tag is typically designed such that the RFID tag can be operated in communication at a long distance. For an RFID tag, thus, a circuit is designed such that receive and response operation is possible with a small power, and the circuit can be operated even with small input amplitude.
In some cases, when an RFID tag is close to a reader/writer, maximum transmission power in conformance with a regulation and the like at place of use may be input to an antenna of the RFID tag. Setting the transmission power of a reader/writer to a certain value varies the reception power of an RFID tag inverse-proportionally with the square of the distance between the reader/writer and the RFID tag. This makes the reception power of an RFID tag that is designed to be operable with a small power very large when the RFID tag is close to a reader/writer. Thus, reception performance of large signals may be unstable or large signal reception may be difficult, disabling communication in wide dynamic range.
In a semiconductor integrated circuit device that is used in an RFID tag and generates a DC power from radio waves received by an antenna, there is proposed a technique to limit output voltage not to increase beyond a certain voltage by providing a voltage limit circuit causing current to flow toward the reference potential when the output voltage output from the output terminal increases beyond the certain voltage (refer to Patent Document 1, for example). In addition, in a device for converting magnetic energy to rectified electrical energy, there is proposed a technique to discharge surplus energy charged in a magnetic field converter circuit through the use of a discharge-promoting circuit (refer to Patent Document 2, for example).
[Patent Document 1] Japanese Laid-open Patent Publication No. 2008-236961
[Patent Document 2] Japanese National Publication of International Patent Application No. 2001-516460